Beware Thinking Like Lamarck

October 26, 2008

Lamarck and Darwin

Jean-Baptiste Lamarck was an 18th century French naturlist who developed the first coherent theory of organic evolution. That is to say, he developed a testable scientific theory of how organisms evolve into other oganisms through time without divine intervention. Lamarck incorporated into his theory the common belief of the day that organisms can inherit the acuired characteristics of their parents. He belived in something he called l’influence des circonstances or the Adaptive Force which causes organisms to adapt to changes in their environment. Those acquired adaptations are then passed on to the offspring of the organism through sexual reproduction. 

Jean-Baptiste Lamarck and Charles Darwin

Jean-Baptiste Lamarck and Charles Darwin

Under Lamarck’s theory, for example, a blacksmith could develop a right arm that was stronger and larger than his left arm. The son of the blacksmith would inherit this characteristic and develop it further and so on and so forth until you get a race of blacksmiths with one giant arm. This is nonsense. An organism cannot pass on acquired traits to its offspring.

In the 19th century, Charles Darwin, showed that evolution can happen without the deus ex machina of an invisible adaptive force. A population of organisms has random variation in traits. All of the organisms cannot survive and reproduce. Only some of the animals successfully reproduce themselves (for whatever reason) and, in the case of species that reproduce sexually, only some of the possible mating combinations occur. This Natural Selection operating on the population is the engine that drives evolution, rather than an Adaptive Force. The other feature of Darwinian evolution is “descent with modification” or the idea that sometimes the offspring in the new generation have a characteristic not found in the parents. We usually call descent with modification  mutation. Mutation and differential reproductive success are the forces that drive evolution.

The thing is, though, that Lamarckian inheritance is very intuitive to people. I often observe people speaking about selection and evolution, especially with respect to purebred dogs, in Lamarckian terms without realizing that they are doing it. Whenever we think about how purebred dogs have changed through time, we need to stop and remind ourselves that animals cannot transmit acquired characteristics to their offspring. We change the characteristics of breeding populations only by selecting which specimens will breed forward to the next generation. Nothing else matters.

Evolution is driven by differential reproductive success. Traits acquired during the life of an animal cannot be passed on to their offspring.

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